1. Field of the Invention
The present invention relates to an apparatus and method for reducing the gradation level of an image by using a plurality of methods.
2. Description of the Related Art
There are proposed a variety of methods as a method for reducing the number of gradations of an image to reproduce an image. For example, error diffusion processing is disclosed in Japanese Laid-open Patent Publication No. 10-032712.
There is also proposed another method for reproducing an image with screen processing. With the method, screens are prepared in advance, and an image is reproduced by using a screen depending on the density thereof.
Reproducing an image with error diffusion processing results in greater sharpness than when an image is reproduced with screen processing, although graininess increases.
In view of this, a method is possible in which a part of the entire image is reproduced with error diffusion processing and the other part is reproduced with screen processing. A technique to selectively use a plurality of methods for reducing the gradation level of an image is disclosed in Japanese Laid-open Patent Publication Nos. 10-032712, 2004-064189, 7-274004, 9-074493, 10-336454, 2002-247356, and 2004-287794.
Unfortunately, the selective use of reproduction methods makes discontinuity of images noticeable, for example, around the boundary between a part of the image reproduced with error diffusion processing and a part of the image reproduced with screen processing. In short, an unnatural connection appears around the boundary therebetween.